The current status of the L5178Y/TK+/- leads to TK-/- mouse-lymphoma mutagenicity assay is described. Dose-survival-mutagenic response data are shown for 43 chemicals. Mutagenicity and cytotoxicity in the presence or absence of non-induced and/or Aroclor-induced rat-liver S-9 are compared for most of these chemicals, 25 of these for which usuable carcinogenicity data exist have been used to construct an approximately linear relationship between oncogenic potency in vivo and mutagenic potency in this system in vitro; linearity between these two endpoints extends over a greater than 100,000-fold range in potencies. Several carcinogens which are negative or difficult to detect in the standard Ames assay are mutagenic in this mammalian cell system. These include natulan, sodium saccharin (lot S-1022), p,p'-DDE (metabolite of DDT), dimethylnitrosamine, diethylnitrosamine and diethylstilbestrol. Characterization of the TK-/- mutants suggests that two mutagenic mechanisms contribute to their final yield. Large-colony TK-/- mutants probably represent point or gene mutations affecting the TK locus. In addition, a class of small-colony TK(/- mutants are described and characterized as being heritably growth-deficient; this and other properties suggest that these small-colony TK-/- mutants originate by a heritable and viable chromosomal aberration. Most carcinogens and mutagens tested produce both classes of TK-/- mutants in this system; the relative proportions of small- and large-colony mutants are both mutagen- and dose-dependent. Comparative studies have been done at the rapidly-expressing TK locus and the slowly-expressing HGPRT locus in these cells. Several carcinogens detected at the TK locus are non- or very weakly mutagenic at the HGPRT locus. This findings is consistent with the induction of slow-growing specific locus mutants by a chromosomal mechanism and their subsequent dilution during this long expression time.